Why do I loose 1st gear and overdrive after warmup

I have a 1996 Olds. Bravada with 4.3 and auto. trans. When I first take off when engine is cool the trans. works great but after the first 3-4 miles when I come to a stop it will not go back into 1st gear and I loose O/D. I just replaced both shift solonoids inside the trans pan but the problem is still there. Is there something else that I can try before committing to a rebuild.